This invention relates to transformer coil bodies, and more particularly, to a transformer coil assembly.
The transformer coil assemblies of interest herein have axially separated winding areas and are typically provided with one or a plurality of coil body or bobbin members which provide coil winding areas. Generally, the bobbin has a core of rectangular cross-section but other cross-sectional shapes such as round or square may be used. Axially spaced flanges extend radially outwardly from the core to define the coil winding areas. The bobbin or bobbins are mounted in a housing or shroud. The bobbin and shroud members are generally formed of molded plastic material which is non-conductive.
The possibility of electrical failure due to the formation of a small electric arc or creep path between adjacent coils has prompted the adoption of standards regarding the length of the minimum creep path. These standards have resulted in the use of the flanges and cooperating insulators to define serpentine or tortuous paths which meet minimum creep path length requirements. For example, U.S. Pat. No. 3,750,072 discloses the use of an insulating frame having an annular sleeve for receiving the flanges of a bobbin and an inwardly projecting annular wall which cooperates with the sleeve to define tortuous creep paths between adjacent coils. The bobbins and frame member are joined in an assembly by entrapment of a further overlying sleeve with a pair of end caps or a pair of side shields. This transformer arrangement requires axial assembly of the bobbins and simultaneous positioning of several pieces prior to joining of the assembly components.
In order to eliminate the manipulation of separate bobbin members during manufacture and/or reduce the total number of parts, one piece bobbin members having axially spaced winding areas separated by flanges are disclosed in U.S. Pat. Nos. 3,909,761 and 4,405,913. In each of these patent teachings, the flanges are provided with grooves in their edges for receiving an insulator extension projecting from an overlying cap member in order to provide a serpentine creep path between the adjacent winding areas. Although the use of a one-piece bobbin reduces the total number of parts and facilitates manipulation during assembly, it does give rise to a tendency of inner flange distortion during winding since it cannot be supported as in the case of separate bobbins. Also, the use of a single bobbin for primary and secondary windings tends to impose greater inventory demands and to limit flexibility of product types for a given inventory.